Decarbonisation of the electrical grid, necessitated by international targets to limit further
global warming, will require a steadily increasing penetration of non-dispatchable
intermittent renewable electricity generation sources. Energy storage has the potential
to substantially increase the grid’s ability to accept greater quantities of renewables
while maintaining stability. Pumped-Heat Energy Storage (PHES) is a form of electrical
energy storage targeted to provide storage on the order of days or weeks, as opposed
to short durations of storage currently available through battery technologies. PHES
systems could be utilised at substations across the country to help the grid endure diurnal
load fluctuations and periods of low wind and solar resource. This system is based
upon the Joule-Brayton cycle, which operates in the reverse direction to store exergy
and the forward direction to generate electricity. An inert gas is the cycle working
fluid, and a liquid is used to transfer heat to and from thermal exergy stores. Exergy is
stored as temperature differences from ambient in balanced hot and cold stores.
PHES development at the University of Edinburgh has iteratively explored different
system architectures, and focused on increasing confidence in components within
these architectures where there is uncertainty with regard to performance. Early work
concentrated on gas-liquid mixing within the cylinder of a compressor/expander machine,
while current work has eliminated such mixing and instead proposes the use of
large scale, direct-contact heat exchangers. Such exchangers suffer from significant
uncertainty for this application owing to the lack of existing experimental correlations
with which to predict their behaviour at the proposed operating pressure and temperature.
As a result, gas liquid surface interactions and heat-transfer between gas and
liquid streams are largely unknown, hindering system development.
Two experimental campaigns were conducted to verify components in both the
early and current system iterations. The first demonstrated a novel in-cylinder gasliquid
mixing device and quantified device behaviour against the no-mix condition.
The second campaign demonstrated operation of a scaled pilot packed-column direct
contact heat exchanger, where gas and liquid comingled to exchange heat. Existing experimental
correlations for high pressure packed column flooding were verified against
experimental results, and the overall heat exchange coefficient was calculated. Results
were used to validate a finite volume heat transfer model based upon previous correlations.
Successful gas-liquid heat exchange in the temperature and pressure range of
interest was demonstrated, advancing PHES development and informing future iterations
of the system
